It seems like there is no 21" TV out there that has a potentiometer for horizontal size.

CRT TV's dropped it by the early 80's.

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The question is: Is horizontal size adjustment needed on a cap that only plays horizontal games?

If your TV displays a broadcast or console signal ok, or nearly ok,the *probably* not. If you're going to be using crt_emudriver you can modify the horizontal porch/pulse timings to adjust the horizontal size. Setup crt_emudriver, then your modelines with VMM, then go into ArcadeOSD and adjust the modelines per info scattered through this thread - http://forum.arcadecontrols.com/index.php/topic,159464.msg1677118.html

Sure. Your website is the reason i collected all this boards and tubes And i remember you where using an Extron and that's the reason i was asking about potentiometers for horizontal adjustments.

That brings me to the question if pots are also needed for other emulators.Nintendo, GameCube, PS, Genesis (consoles) emulation shouldn't need it because this consoles where made for standard TV's - right?

I have already planned 2 caps and have a Loewe for the old consoles.Would be nice i can use the Loewe also for console emulation.

That brings me to the question if pots are also needed for other emulators.

Maybe... if the emulator isn't super-resolutions aware (i.e. won't do horizontal scaling) then you'd have to use a standards mode and the modeline centering isn't as granular for those.

A while back I explored automating an Extron Rxi via serial port control. I managed to get an AutoHotkey script communicating with the Extron (e.g. query the current H and V position values, measured refresh rate and horizontal frequency, remotely set the H and V position, etc).

Idea would be to use AutoHotkey to monitor for changes in video mode and application focus then set the H and V positions accordingly.

I think this is entirely possible but I moved onto a more important project (using an Arduino controlled relay array to created a digital voltage divider that automatically sets the correct vertical size for each game in GroovyMAME).

For that last project, I have a working prototype (16 preset sizes only currently but could be expanded with more relays) but haven't had a time to write it up and share here (in the process of moving house).